As disclosed by US 2015/0045184 A1, there is a well-known conventional four-wheel driving vehicle, called “utility vehicle” (hereinafter referred to as “UTV”), equipped with a power train including an engine and front and rear transaxles. The engine and the rear transaxle are disposed at a rear portion of the UTV. The rear transaxle carrying right and left rear wheels is drivingly connected to the engine via a belt type continuously variable transmission (hereinafter referred to as “CVT”), for example, so as to receive power from the engine via the CVT.
The rear transaxle further includes a power take-off (hereinafter referred to as “PTO”) unit so as to distribute the power from the engine to the right and left rear wheels and the PTO unit. The front transaxle carrying right and left front wheels is disposed at a front portion of the UTV, and includes an input shaft for receiving power from the PTO unit of the rear transaxle so as to distribute the power to the right and left front wheels.
A driving mode selection clutch is provided on the input shaft of the front transaxle. The driving mode selection clutch is selectively engaged to transmit power from the PTO unit of the rear transaxle to the right and left front wheels, or disengaged to isolate the right and left front wheels from the power from the PTO unit.
The drive mode selection clutch is manually operable so that the selection of whether the UTV travels in a four-wheel drive (hereinafter, referred to as “4WD”) mode or a two-wheel drive (hereinafter, referred to as “2WD”) mode depends on a driver's option.
The UTV is often used to travel over undulating terrain. In this case, a driver tends to set the UTV in the 4WD mode so as to ensure running performance by driving all of the four wheels. However, during such a traveling, the UTV may repeat jumping and landing.
When the UTV lands, falling and landing of the front wheels precedes falling and landing of the rear wheels. Especially, the moment the UTV lands by only the front wheels preceding the rear wheels, a heavy load from the earth is suddenly applied to the front wheels, and backflows to stress the entire power train.
Therefore, it is desirable that when the UTV jumps, the engaged drive mode selection clutch is instantly disengaged to make the front wheels free from the driving power from the PTO unit before the vehicle lands with the front wheels.
However, it is difficult for a driver to make such instant decision and operation to disengage the clutch each time the UTV jumps. Here, it may be conceivable that the UTV is equipped with an electronic control system for automatically controlling an actuator for switching the drive mode selection clutch based on detection of jumping and landing of the vehicle, however, such a system will be complicated and expensive.